Seems to me, that for what you describe, human error could not be a factor because that implies (at least to me) fore knowledge (not speculation or educated guesses) that was used incorrectly. The Pioneer's are in, literally, uncharted territory, and therefore no knowledge of the conditions existed from which to err.
And the Pioneer's probably don't have the instrumentation and or power to investigate, so such knowledge will still be lacking until we send out more extra-solar spacecraft to specifically study the phenomena.
Seems to me, that for what you describe, human error could not be a factor because that implies (at least to me) fore knowledge (not speculation or educated guesses) that was used incorrectly. The Pioneer's are in, literally, uncharted territory, and therefore no knowledge of the conditions existed from which to err. Well, if that is the case, then the very premise of predicting the theoretical velocity of the craft, to compare with the actual velocity, should be an erroneous proposition, shouldn't it?